This invention relates generally to universal motors, and more specifically to a method for winding a field assembly for a universal motor having brake windings and an apparatus produced thereby.
Universal motors (i.e., those capable of being powered by either direct current (DC) or alternating current (AC)) have been widely used in home appliances and industrial machines for many years. FIGS. 1A and 1B show a conventional universal motor usable, for example, in a miter saw. The universal motor is comprised of two main components: the field assembly 30 and the armature 20. The armature 20 comprises: a shaft 21; a commutator 22 including several commutator bars or contacts 35; a plurality of armature windings 23; an armature core including a plurality of armature poles 24 over which the armature windings 23 are wound; a fan 25; and bearings 26. The field assembly 30 comprises: a field core 31; field windings 32; brake windings 34; field poles 36 (not visible) over which the field windings 32 and the brake windings 34 are wound; two connector tabs 38 connected to the end leads of the field windings 32 (only one is shown); and two connector tabs 39 connected to the end leads of the brake windings 34 (only one shown). The connector tabs comprise a portion of the terminal boards which are affixed to the two flat ends of the field core 31. Also shown in FIG. 1B are the brushes 27 which contact the bars 35 on the commutator 22 to energize the armatures windings 23. When the armature windings 23 and the field windings 32 are energized by the application of current to the brushes 27 and the connector tabs 38 respectively, the armature 20 turns inside of the field assembly 30.
FIG. 2 shows a circuit diagram of the conventional universal motor of FIG. 1A and 1B. The windings are connected such that, when the motor is engaged by connecting the switch 37 to terminal A, the field windings 32 and the armature windings 23 are connected in series and current passes through them. The current flowing through the field windings 32 causes a north magnetic pole and a south magnetic pole to form on the two poles 36 over which the field windings 32 are wound (see FIG. 3), thus creating a substantially parallel magnetic field between the two poles 36. The brushes 27 and the commutator 22 route this current through the armature windings 23, causing another magnetic field to form around the armature 20. Because the armature windings 23 are wound in a manner to create a magnetic field which is approximately 90 degrees from the magnetic field produced by the field windings 32, the two fields will attempt to align, and the armature 20 will turn inside of the field assembly 30. However, when the armature 20 is turned, a new set of bars 35 on the commutator 22 are energized, and a new approximately 90 degree magnetic field is established by the armature windings 23 in the armature 20. In this way, the armature 20 will continually spin within the field assembly 30 when energized.
Also shown in the circuit of FIG. 2 are the brake windings 34. The brake windings 34 are used to quickly slow or "brake" the motor when power to the field windings 32 has been disengaged. Specifically, when the power to the field windings 32 is disengaged by moving the tap of the switch 37 to terminal B, the field windings 32 and the armature windings 23 are disconnected, which causes the armature 20 to "coast" to a stop. In addition, connecting the switch 37 to terminal B routes the brake windings 34 in series with the armature windings 23. Residual magnetism in the field core from the previously energized field windings 32 causes the still spinning (i.e. "coasting") armature 20 to generate a small current through the brake windings 34 and the armature windings 23. Because the brake windings 34 are now energized by the small current, they too will create a magnetic field. However, because the brake windings 34 are wound in the opposite direction of the field windings 32, the magnetic field produced by the brake windings 34 will be opposite of the field previously formed by the field windings 32. The small current provides positive "feedback" which increases the magnetic field produced by the brake windings 32, thus producing an even higher current, and so on, until the current becomes limited by the resistance of the brake windings 34 and the armature windings 23. In this way, the mechanical energy of the spinning armature 20 is extracted through the brake winding 34/armature winding 23 circuit, thus quickly braking the motor. By designing the brake windings 34 to have an appropriate resistance, the rate at which the motor brakes can be well controlled, with the extracted mechanical energy being dissipated as heat in the brake windings 34/armature winding 23 circuit. The operation of universal motors is well known to one of ordinary skill, and the reader is directed to the "Standard Handbook for Electrical Engineers," 10th Edition, McGraw-Hill Book Company, Section 7, Section 18 (paragraphs 1, 125 and 127), and Section 21 (paragraphs 211 and 212) (1969), which is incorporated by reference herein in its entirety.
FIGS. 3A-3C show the field assembly 30 in more detail (with the armature 20 removed for clarity). As previously noted, the field windings 32 and the brake windings 34 are wound around each field pole 36 of the field assembly 30. Traditionally, these windings 32 and 34 are wound in two steps and on two separate winding machines. Typically, the field winding 32 is wound first using a first machine. Then the partially wound field assembly 30 is transferred to a second machine which winds the brake windings 34 over the field windings 32. It is also known to wind the field windings over the brake windings. This conventional practice requires two machines, involves additional labor to transfer the partially wound field assembly 30 between machines, and results in a field assembly 30 with eight leads: two leads for each of the field windings 32, and two leads for each of the brake windings 34. A field winding lead on the first pole needs to be spliced together with the field winding lead on the second pole (referred to as "crossover connections" 41) to connect the two field windings 32 in series (the brake windings 34 are similarly connected). FIG. 3A shows the crossover connection 41 for the field windings 32 which appears between the two poles 36 (the crossover connection 41 for the brake windings 34 appears on the opposite side of the field assembly 30 and is seen in phantom in FIG. 3A). The windings 32 and 34 can be connected at the crossover connections 41 by solder or by "crimping," but other means of making a connection are suitable. However, connecting the crossover connections 41 adds to the number of steps required to assemble the field assembly 30, further increasing production costs. Additionally, the crossover connections 38 occasionally fail, hampering the reliability of the finished motor product.
Another way of braking the motor is to use two pole switches so that the field windings and the brake windings are both connected in a generator mode during braking. Such an approach is used in commercially available miter saws such as the Sears/Craftsman.TM. 10-inch Compound Miter Saw, Model No. 113.234600, sold by Sears Roebuck & Co., Chicago, Ill. 60684. The Owner's Manual of this miter saw product shows a circuit diagram of this arrangement (pg. 23), which is incorporated by reference herein in its entirety. This arrangement requires the use of two normally-open contacts and two normally-closed contacts which, during braking, reverse the connections of the field winding and connects the reversed field windings in series with both the brake windings and the armature windings (so that the resistance of the field winding may be used to assist in braking the motor). While such an approach has been shown to produce adequate braking characteristics, it requires a complicated arrangement of components within the motor.
The present invention improves on these prior art practices by disclosing an improved field assembly winding method and field assemblies produced thereby.